ds had a cartridge slot on the bottom for gameboy games...
I never played the ds, but I'm glad I didn't in that case. The last handheld I had before I got the 3ds was the Gameboy Advance.
Was it the same sort of cartridge technology, though? With how easily it pops in and out, I mean? Cause everything before that (I can't speak for the Gameboy SP personally, but I think it had the same cartridge technology and location as its predecessors) never popped out that easily. You had to actually pull it out, and if you drop it, you'd have more of a chance of the battery case popping off as a minor inconvenience than the game popping out (actually, never had the game pop out on me with the GBA and Pocket).